This invention generally relates to electric insulation materials. More particularly, this invention is directed to sheet materials for forming groundwall insulation of a high voltage generator stator bar, in which oxide-filled sheet material is used in combination with oxide-free sheet material, yielding a stator bar having significantly improved voltage endurance performance.
Polymer-impregnated sheet materials, such as fabrics, films, paper and tapes, have been widely employed to form electrical insulation for various electrical equipment and components, including high voltage stator bars of generators. Formation of such insulation generally involves the use of a pre-impregnated sheet material, often referred to as a prepreg, that can be applied directly to a member to be insulated. Various materials can be employed as the sheet material and the impregnation material, depending on the requirements of the application.
Referring to FIG. 1, a stator bar 10 for a generator is represented that is illustrative of high voltage stator bars known in the prior art. As shown, the stator bar 10 is composed of a number of conducting copper strands 12 that are insulated from each other by strand insulation 13. The strands 12 are arranged to form two tiers that are separated by a strand separator 14. Surrounding the tiers is a groundwall insulation 15 formed by multiple wrappings of a mica paper tape 16.
As illustrated in FIGS. 2 and 3, the mica paper tape 16 is a prepreg composed of a mica paper 17 backed by a single woven backing 18 as shown in FIG. 2, or a pair of backings 18a and 18b as shown in FIG. 3. In the configuration of FIG. 3, one of the backings 18a or 18b can be a woven fabric such as fiberglass while the second can be another woven fabric, a nonwoven fabric such as a polyester mat, or a polymer film such as polyester, polyimide or polyetherimide film. In each case, a resin composition is used to permeate through the mica paper 17 and to bond each backing 18, 18a and 18b to the mica paper 17, thereby forming the prepreg tape 16.
Various resin compositions can be used to impregnate the mica paper 17 and backings 18, 18a and 18b, an example of which is evidenced by U.S. Pat. No. 3,563,850 to Stackhouse et al., assigned to the assignee of this invention. Stackhouse et al. teach mica tapes for electrical insulation using a solventless epoxy-based resin composition. Prepregs of the type taught by Stackhouse et al. are typically slit into tapes that can be more readily wrapped around a stator bar of a generator. Typically, multiple layers of tape are tightly wrapped around the conductor, usually overlapping by one-half the width of the tape, or xe2x80x9chalf-lapped.xe2x80x9d After being wrapped with a sacrificial release film to protect the tape and prevent contamination, the conductor and its tape wrapping are then placed in an autoclave for vacuum heat treatment and subsequent curing. Vacuum heat treatment is carried out to remove air, moisture and any solvent or volatile compound present in the resin binder, while curing under pressure serves to consolidate the tape insulation, such that the resin binder bonds the mica paper and each of its backings together to form a void-free solid insulation.
Removal of air, moisture, solvents and volatile compounds from the binder is necessary to prevent formation of voids in the cured insulation that would otherwise adversely affect the quality of the insulation and induce premature insulation failure due to breakdown under electrical stress. The latter characteristic of insulation is termed xe2x80x9cvoltage endurance,xe2x80x9d and is quantified as the time-to-failure or breakdown of the groundwall insulation under high voltage electrical stress. Voltage endurance breakdown is normally due to erosion by electrical discharge and electrochemical attack. By conducting voltage endurance tests at different voltages, a relationship of life versus stress can be determined for a specific insulation system.
Improvement of voltage endurance performance of high voltage generator stator bars is a continuous and desirable objective because of product reliability and machine performance. Improvements in voltage endurance performance have been achieved with synthetic, thermosetting polymeric materials such as epoxy and polyester resins. Further improvements in voltage endurance have been made through further refinements in the resin binder, such as modified or new resin compositions and hardener chemistries, as taught by Stackhouse et al. Nonetheless, further improvements in voltage endurance are desirable.
It is an object of this invention to provide an insulation material that exhibits enhanced voltage endurance.
It is a further object of this invention that such an insulation material is suitable as groundwall insulation for a high voltage generator stator bar.
It is another object of this invention that such an insulation material is formed by layers of resin-rich sheet material, wherein at least one of the layers is formed by a sheet material containing submicron oxide particles while at least one other is free of such oxide particles.
The present invention provides resin-impregnated sheet materials, such as fabrics, films, paper and tapes, to form electrical insulation that exhibits significantly improved voltage endurance performance. While the present invention finds particular use with conductors that are insulated with resin-rich tapes processed by press-curing or autoclaving, in which a taped conductor is first vacuum and heat treated to remove air and volatile components and then compacted and cured by applying heat and pressure, this invention is also applicable to vacuum-pressure impregnation processes where two or more tapes are wrapped around a conductor and then the taped conductor is vacuum-pressure impregnated with a solventless, low viscosity resin.
According to this invention, the voltage endurance performance of a high voltage generator stator bar is improved by forming the groundwall insulation using a mica tape filled with submicron particles of silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium dioxide (TiO2) or zirconium dioxide (ZrO2), in combination with an unfilled mica tape, in which the tapes are impregnated with the same or compatible resin binders. More particularly, the filled tape is in the form of an electrical insulating sheet comprising a mica paper having at least one woven fabric on at least one of its surfaces, a resin binder permeating the mica paper and woven fabric so as to bond the woven fabric to the mica paper, and oxide particles dispersed in the woven fabric. As noted above, the oxide particles are silicon dioxide, aluminum oxide, titanium dioxide and/or zirconium oxide. In addition, the oxide particles preferably have a particle size of about 0.005 to about 0.05 micrometers. The unfilled tape is also in the form of an electrical insulating sheet of mica paper having at least one woven fabric on at least one of its surfaces, and a resin binder permeating the mica paper and woven fabric so as to bond the woven fabric to the mica paper. However, the unfilled tape is essentially free of oxide particles.
According to this invention, a significant improvement in insulation life has been unexpectedly found for groundwall insulation formed by the combination of filled and unfilled mica tapes of this invention. In addition, this combination of tape materials appears to have a synergistic effect because the combination exhibits significantly improved results as compared to the individual materials when used alone.
Other objects and advantages of this invention will be better appreciated from the following detailed description.